1. Field of the Invention
The present invention relates to golf balls. More particularly, the present invention relates to golf balls having a mid layer comprising a crosslinked rubber.
2. Description of the Related Art
Golf balls used for playing golf at a golf course are generally classified into: wound golf balls having a core comprising wound rubber threads; and solid golf balls (two-piece golf balls, three-piece golf balls and the like) having a core comprising a solid rubber. Wound golf balls have been conventionally used, with a period during which the wound golf balls account for almost all of the first-class golf balls. However, solid golf balls that have been developed afterwards can be readily manufactured at a lower cost, therefore, larger number of solid golf balls have been recently supplied to the market than the wound golf balls. In general, solid golf balls have drawbacks in a feel at impact being hard. To the contrary, common solid golf balls are more excellent in terms of a travel distance than wound golf balls.
In an attempt to improve a feel at impact or to further enhance a flight performance, solid golf balls including a core having two layers, i.e., a center and a mid layer, have been proposed and placed on the market. Half shell methods are often adopted to produce such golf balls. In a half shell method, dome shaped half shells are first formed from a rubber composition for the mid layer. Next, a center is covered with two half shells. These center and half shells are placed into a spherical cavity of a mold, and then the mold is clamped. Upon clamping of the mold, excess rubber composition outflows from a parting face of the mold. A mid layer is thus formed. Processes for producing golf balls according to such a half shell method are disclosed in Japanese Patent Publication References JP-A-24124/1998, JP-A-99470/1998 and JP-A-108921/1998.
Meanwhile, a variety of techniques have been proposed where particles (solids) comprising a crosslinked rubber or a synthetic resin are blended in a core of a golf ball. For example, a core blended with particles of ebonite that is a highly hard rubber is disclosed in Japanese Patent Publication Reference JP-A-94666/1986. In Japanese Patent Publication Reference JP-A-91019/1994, there is disclosed a core blended with high-molecular weight polyethylene (trade name: “Miperon XM220”) having a shore D hardness of about 65. In Japanese Patent Publication Reference JP-A-185039/1995, there is disclosed a golf ball with mitigated impulsive force at impact through blending vulcanized rubber powder having a particle diameter of from 0.8 mm to 7.0 mm in a core. In Japanese Patent Publication Reference JP-A-314342/1998 (U.S. Pat. No. 5,779,562), there is disclosed a golf ball having a central core layer, an outer core layer, an inner cover layer (This inner cover layer can be also assumed as an outermost layer of the core.) and an outer cover layer, wherein polypropylene powder is blended in the core.
In Japanese Patent Publication Reference JP-A-583/2001 (U.S. Pat. No. 6,361,453), there is disclosed a golf ball having a core blended with particles of which hardness being higher than the hardness of the core surface. In Japanese Patent Publication Reference JP-A-584/2001 (U.S. Pat. No. 6,361,453), there is disclosed a golf ball having a core blended with particles whose difference from the core being small in their specific gravity. In Japanese Patent Publication Reference JP-A-587/2001 (U.S. Pat. No. 6,361,453), there is disclosed a golf ball having a core blended with particles, without exposure of these particles to the core surface. Japanese Patent Publication Reference JP-A-29511/2001 discloses a golf ball having a mid layer including rubber particles dispersed in a thermoplastic resin.
When the mid layer is formed by the half shell method described above, the center is neither supported nor immobilized. Therefore, the center may be migrated within the cavity upon streaming and/or outflowing of the rubber composition for the mid layer, and thus positional shift may occur between the central part of the center and the central part of the cavity. When the positional shift occurs, deviation of mass of the mid layer (uneven wall thickness) arises, thereby resulting in varying extent of spin, launch angle, initial velocity, travel distance, trajectory and feel upon impact of the golf ball in dependence upon the hitting point. Deviation of mass also adversely affects on durability and straight flight performance of the golf ball.